Recent developments of the 3GPP Long Term Evolution (LTE) facilitate accessing local IP based services in the home, office, public hot spot or even outdoor environments. One of the important use cases for local IP access and local connectivity involves the direct communication between devices in the close proximity (typically less than a few 10s of meters, but sometimes up to a few hundred meters) of each other.
In the following, the term user equipment (UE), unless explicitly stated otherwise, may refer to any terminal, e.g. mobile terminal, or wireless device, which may be adapted for cellular communication/operation and for D2D communication/operation. A user equipment may be a mobile phone, smartphone, tablet, laptop, computer, sensor arrangement, smart device, etc. adapted for such communications or operations. A user equipment may provide a terminal point or end point for communication using cellular operation and/or D2D operation. Generally, a UE may be ProSe enabled (or D2D enabled), adapted for ProSe/D2D communication/operation, e.g. according to a given standard like LTE.
This direct mode or device-to-device (D2D, also called ProSe for Proximity Services, in particular in the context of LTE, or sidelink) enables a number of potential gains over the traditional cellular technique, because D2D devices are much closer to one another than cellular devices that have to communicate via one or more cellular access points (e.g., a radio network node or eNB):                Capacity gain: First, radio resources (e.g. OFDM resource blocks) between the D2D and cellular layers may be reused (reuse gain). Second, a D2D link uses a single hop between the transmitter and receiver points as opposed to the 2-hop link via a cellular AP (hop gain).        Peak rate gain: due to the proximity and potentially favorable propagation conditions high peak rates could be achieved (proximity gain);        Latency gain: When the UEs communicate over a direct link, eNB forwarding is short cut and the end-to-end latency can decrease.        